tools: transport-proof demand writes + tolerant demand parsing (the pool-pin latency root cause)

The r581 boot's guest scanner died mid-boot with ValueError in main():
"invalid literal for int() with base 16:
'0x20400f000\\n0x121000000\\n0x12000ab4c'". A guest-agent transport-escaped
append merged demand VAs onto one line with literal backslash-n, and the
old parser crashed on it. The crash stopped ALL new pins; the host only
re-asserted known pairs, so the UVM pool VA 0x121000000 never arrived
while 185 other injections flowed - the pool-pin latency that blocked
every validation boot.

* pin_gr_stream._demand_send/_demand_loop now write one "echo <va> >>"
  per VA (no newlines, no backslashes in the transported command; immune
  to transport escaping).
* guest_pin_stream's demand parser extracts every 0x token with re and
  skips malformed ones instead of raising out of main().

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